This invention is relative to an X-ray examination apparatus, comprising
an X-ray source,
an X-ray detector,
a filter arranged between said source and said detector, said filter comprising an array of filter elements having X-ray absorbtivities that can be adjusted by means of control voltages,
a control circuit for supplying said control voltages to said filter elements, and
an object support arranged between said filter and said detector, said station being adapted to support an object to be exposed to X-ray radiation emanating from said source, the transmitted X-ray radiation being detected by said detector.
Such an apparatus is known from e.g. U.S. Pat. No. 5,625,665.
This prior art reference is relative to a dynamic beam attenuator, which is a pixelwise adjustable X-ray filter, by means of which parts of a patient to be examined can be effectively covered so that these parts are not unnecessarily exposed to X-ray radiation. This defines a Region of Interest or ROI. Thus the radiation dose to which the patient is exposed is decreased and the influence of scattered radiation is diminished. This prior art principle is also applied in so-called slit-scanning. A small slit is formed by the dynamic beam attenuator and is moved effectively over the patient in order to form a total X-ray picture of the patient. Outside the slit radiation of a different spectral composition is transmitted. If desired, more slits can be used simultaneously in order to decrease the effective scanning time, which of course goes at the expense of the reduction of scattered radiation.
Due to the reduction of scattered radiation due to slit-scanning the contrast in the picture as finally obtained improves.
Of course the effective electrical power load imposed on the X-ray source is higher in the case of slit-scanning. A further disadvantage may be residing in the fact that the discrete slits will be noticeable in the final picture.
It is a purpose of the invention to provide an apparatus that allows making an X-ray picture of an object, e.g. a patient, within a time frame of about one second.
It is a further purpose of the invention to design an apparatus of the kind set forth in the way such that the influence of scattered radiation is reduced.
Generally, the X-ray examination apparatus comprises said control circuit being adapted to supply said control voltages in single-sequence fashion to groups of adjacent filter elements.
It should be noted that this invention is not limited to the technique according the mentioned prior art reference U.S. Pat. No. 5,625,665, in which the filter elements each include a capillary tube communicating with a reservoir with an X-ray absorbing liquid, the electrical control taking place by controlling the capillary properties of said capillary tubes.
The apparatus according to the invention can advantageously be designed such that said groups are evenly and regularly distributed over the filter.
The basic principles according to the invention described herein above can be implemented in several technical ways.
In a practical mechanical embodiment each filter element comprises an X-ray absorbing element coupled with an actuator controlled by a respective control voltage, thus controlling the effective X-ray absorbtivity of said filter element.
This embodiment can be designed such that said X-ray absorbing element comprises a heavy element, e.g. lead.
The mechanical actuator may be adapted to cause the associated filter element to follow a specific linear or curved path.
In an alternative embodiment the filter element comprises a liquid crystal element controlled by a respective control voltage for controlling the effective X-ray absorbtivity of said filter.
In order to ensure sufficient X-ray intensity attenuation the liquid crystal element should have sufficient thickness or the filter element may be composed of a plurality of liquid crystal elements.
A preferred embodiment is embodied such that each filter element comprises a capillary tube connected to a reservoir for X-ray absorbing liquid, the inner surface of said capillary tube at least partly being coated with an electrically conductive layer connected with said control circuit for receiving a respective control voltage for adjusting the amount of X-ray absorbing liquid present in said capillary tube thus controlling the effective X-ray absorbtivity of said filter element. The filter structure is known per se from U.S. Pat. No. 5,625,665. The novel feature according to the invention is residing in the specific mono-cyclic control such that spot-scanning occurs.
With a view to designing the apparatus according to the invention in a way such that an extremely high signal to noise ratio is achieved a preferred embodiment further comprises a signal processing assembly receiving detector signals from said detector, said detector signals being group-wise arranged in accord with the supply of said control voltages to said groups of adjacent filter elements, said groups of detector signals being supplied to a memory means, said signal processing assembly being adapted to reconstruct an image by comparing pixel-wise said respective groups of detector signals stored in said memory means and using only every pixel value which is larger than the signal values of the corresponding pixel of every other group.
These and other aspects of the invention will be apparent and elucidated with reference to the embodiments described hereinafter.